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Is there a way to tell if a corrector plate is on backwards in a sct scope (c8)

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#26 Starman1

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Posted 05 June 2017 - 12:58 PM

Some notes from overhauling many of these over the years:

 

1) The corrector should be centered in its cell (using the shims), but ONLY if the primary mirror is centered in the tube.

I used to assume the primary was centered, but on some SCTs, it wasn't (!).  The corrector must be centered on the primary, so check that first.

 

2) The secondary should be centered within the corrector.  Since the secondary mirror may not be accurately centered in its holder, you cannot tell this

unless the corrector and secondary are removed to note the offset if there is any.  The secondary holder may need to be a bit eccentric in the corrector if the secondary mirror

is off center.  If the secondary is more than a millimeter off-center, I'd seriously contemplate detaching it and reattaching it accurately centered.

 

3) The secondary should also be centered on the primary (see note #2) but ONLY if the primary baffle is in the tube completely straight, i.e. is parallel to the tube.

You cannot assume that it is.  I have seen many scopes where the primary baffle did NOT point at the secondary mirror exactly and one side of the baffle clipped

the light cone more than the other.  The difference of a millimeter isn't important, but I had a couple that had to be returned to the manufacturer to align the primary baffle because

the movement of the primary mirror was not coincident with the line of the tube wall due to a tilted baffle.

 

4) It is unlikely the exact center of the secondary will reflect a laser beam back at its source when/after the scope has been star-collimated.

Even the tiniest of tilt corrections in the secondary due to its being a trace off-center will return a laser beam to a different point.  This is one reason NOT to use a laser for

secondary collimation.  The daylight collimation test http://www.robincasa...ro/collimation/  only works if everything is centered and lined up.  Otherwise, it merely gets you

close enough to use a star for collimation.  Checking collimation on a star (artificial or real) is critical for best results.

 

5) The inside surfaces of correctors all develop a haze after a while and need to be cleaned.  This is easiest if the corrector + secondary is removed and placed back in the scope backwards.

Then the inside is easy to clean.

 

6) Rotation of the corrector relative to the primary should be maintained unless you're willing to experiment.  Slight eccentricities in secondary placement sort of demand it.

If the corrector and cell do not have registration marks under the pressure ring that holds the corrector in place, make your own with a Sharpie pen or white-out right at the edge of the corrector.

The mark should be small, so it will be hidden by the pressure ring when it is replaced.



#27 yukosteel

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Posted 05 June 2017 - 03:30 PM

Thank you so much Starman1, that's so helpful guide! That daylight collimation link is very clear, I've seen something like picture 2, just different shift directions.

I've assumed that "arrow 4" on my corrector indicates 4 o'clock position, so I rotated it to that point. Then I centered corrector first but had to move it little down off center, so looks like the primary is also a little shifted. Finally I've moved secondary frame (with removed secondary) to make it centered.

Last night I made star collimation, and it looks ok now : )



#28 Zygmo

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Posted 08 July 2017 - 10:21 PM

Ok. My 2 cents worth.

 

I have a couple of spare C8s that I experiment with. When this d--- hot weather breaks, a friend and I are going to do a more careful study of these things. But, the one thing I have already done is the following.

 

I happened to have the corrector off one of them for cleaning one day, and I decided to try spinning it to different positions to see what, if any, changes occurred. This was done in the daytime, with a stop sign a block away to view. So this is just what I found visually.

 

I placed the corrector ring back on, just tight enough to let the corrector spin, by holding the front of the secondary mirror cell. So keep in mind that the secondary was turning with the corrector.

 

Starting in the original position, with the corrector etching at 3:00, the view was fine. Spinning it clockwise, the view worsened the closer I got to 6:00, then started back to being right with the etching at 9:00. Continuing the spin, it got worse by 12:00, and then the view again became perfect when I got back to 3:00. In other words, it is bad at 90 degrees off either way, but good at 180 degrees with the etching exactly opposite from original position. By the way, at 6:00 and 12:00, the view was obviously distorted.

 

Celestron claims that they made any necessary figure corrections on the secondary mirror. If this is true, and both mirrors are suppose to be figured spherical, that would explain the changes at 90 degrees....right?

 

When we get back to these experiments this fall, we will try spinning the corrector plate, while leaving the secondary mirror in it's correct orientation. I am betting that moving just the corrector will not visually change the view. We will also flip the corrector front to back to see what happens, and do some corrector swapping between scopes, etc.



#29 rmollise

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Posted 09 July 2017 - 10:49 AM

I've moved out 8SE corrector yesterday, and didn't locate any serial number on it.

I also think it doesn't make any difference because the glass is flat and coating looks same on both sides.

 

The position of the corrector and the secondary making difference though. When they are both shifted to the side, the secondary collimation is more tough.

But that's just my personal experience.

 

The corrector is most assuredly not flat. ;)

 

As has been said, which side faces out is of no consequence.

 

What matters most is the radial orientation and proper centering of the secondary. Being centered on the tube is not always the correct position for the secondary, either.



#30 Gil V

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Posted 09 July 2017 - 11:28 AM

OMG another one of these...

#31 Zygmo

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Posted 09 July 2017 - 11:59 AM

OMG another one of these...

What, Apollo? Have these experiments been posted here before? I have seen plenty of posts about the orientation of the individual parts, but no actual reports of trying it. If so, I won't waste my time posting my results later. I personally would like to know if you actually can swap corrector plates with no decrease in quality, for instance.

 

Dean



#32 Starman1

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Posted 09 July 2017 - 04:40 PM

SCTs have been made two ways:

 

Make a whole bunch of mirrors and a whole bunch of correctors and a whole bunch of secondaries.

Put them together and see if it works.  If not, change one of the components (usually the secondary).

Meade did this with most of their SCTs.

 

or

 

Make the mirrors and corrector, put them together, figure out what changes have to be made to the optical surfaces to yield a better image,

pull the scope apart and do hand figuring on the secondary mirror and reassemble.

Celestron did this with their earlier SCTs made in the US.  Since going to China, I doubt this happens.

 

Both techniques have worked.

 

And IF:

--the baffles are exactly perpendicular to the rear cell plate (check with a sight tube)

--the primary is centered in the tube (remove from cell and check)

--the corrector is centered in its cell (easy to measure)

--the secondary cell is centered in the corrector (easy to measure)

--the secondary is centered in its cell (easy to measure)

THEN:

--the image quality will not change with rotation of the corrector, secondary cell, or secondary.

--a laser beam from the rear opening will return exactly on itself.

 

What are the odds?

I don't know, but I haven't yet seen an SCT in which the IFs are all correct.

 

Here's the thing, though.  If the scope is made like my first technique example, you can make all the corrections yourself and improve the images.

If it's made like the second example, everything may need to be exactly as is to yield the best images.



#33 freestar8n

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Posted 09 July 2017 - 05:01 PM

No optical systems made by humans are perfect surfaces with no asymmetries - so there will always be a preferred orientation of the elements even if they are intended to be rotationally symmetric.  The only question is what difference it actually makes and if it is worth worrying about.

 

For a mass produced sct it makes sense to save money by not spending too much time on figuring the elements perfectly - and instead use the freedom to orient the elements to cancel out figuring errors as much as possible.

 

It's clear from the markings on the sct elements that they have a preferred orientation - and if the corrector isn't symmetric then the performance will be different when you flip it over - and once flipped it may have a different, preferred orientation with respect to the other elements.

 

But I doubt it makes much difference or is worth worrying about - as opposed to getting the angles of the elements right - which appears to matter more.  This isn't a big surprise since errors in two surfaces could cancel in one angle orientation - and add in a different one.  That could be a big difference even if those errors aren't too big.

 

Frank



#34 GlennLeDrew

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Posted 09 July 2017 - 06:30 PM

Ok. My 2 cents worth.

 

I have a couple of spare C8s that I experiment with. When this d--- hot weather breaks, a friend and I are going to do a more careful study of these things. But, the one thing I have already done is the following.

 

I happened to have the corrector off one of them for cleaning one day, and I decided to try spinning it to different positions to see what, if any, changes occurred. This was done in the daytime, with a stop sign a block away to view. So this is just what I found visually.

 

I placed the corrector ring back on, just tight enough to let the corrector spin, by holding the front of the secondary mirror cell. So keep in mind that the secondary was turning with the corrector.

 

Starting in the original position, with the corrector etching at 3:00, the view was fine. Spinning it clockwise, the view worsened the closer I got to 6:00, then started back to being right with the etching at 9:00. Continuing the spin, it got worse by 12:00, and then the view again became perfect when I got back to 3:00. In other words, it is bad at 90 degrees off either way, but good at 180 degrees with the etching exactly opposite from original position. By the way, at 6:00 and 12:00, the view was obviously distorted.

 

Celestron claims that they made any necessary figure corrections on the secondary mirror. If this is true, and both mirrors are suppose to be figured spherical, that would explain the changes at 90 degrees....right?

 

When we get back to these experiments this fall, we will try spinning the corrector plate, while leaving the secondary mirror in it's correct orientation. I am betting that moving just the corrector will not visually change the view. We will also flip the corrector front to back to see what happens, and do some corrector swapping between scopes, etc.

While doing this spinning of the corrector--with secondary being carried along for the ride--one or both of secondary translation and tilt will result in mis-collimation. It might have been the case that for each rotational increment if the collimation had been addressed the image quality would have been improved, perhaps to an acceptable degree.



#35 Zygmo

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Posted 09 July 2017 - 09:55 PM

Besides the fact that it cleared up at 180 degrees, I would say the distortion did not look like bad collimation. But actually my main interest is finding out if correctors can be successfully interchanged. Maybe in a couple of months I can come up with some more information, regardless of how unscientific it will be. :)



#36 Cepheus Elf

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Posted 10 July 2017 - 06:32 AM

I'm no optical expert, so I'my just trying to get my head around the fact that people are saying it doesn't matter which way around the corrector plate sits. If it's there to "correct" won't turning it round make it do the opposite and make the image poorer?

 

Cheers,

Mick 



#37 freestar8n

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Posted 10 July 2017 - 07:57 AM

The plate is nearly flat so it "corrects" just by slowing down the light a bit in the thicker regions relative to the thinner ones.  The thickness is the same when you flip it over - so it still works.

 

There would be a tiny difference in how the light actually behaves so it is indeed optically different when flipped - even if it were made perfectly - when one side is flat and the other is curved.  But it's such a small effect it is negligible.

 

Frank



#38 Cepheus Elf

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Posted 10 July 2017 - 08:02 AM

The plate is nearly flat so it "corrects" just by slowing down the light a bit in the thicker regions relative to the thinner ones.  The thickness is the same when you flip it over - so it still works.

 

There would be a tiny difference in how the light actually behaves so it is indeed optically different when flipped - even if it were made perfectly - when one side is flat and the other is curved.  But it's such a small effect it is negligible.

 

Frank

That makes sense to me now Frank, thanks for the explanation!


Edited by Cepheus Elf, 10 July 2017 - 08:13 AM.


#39 SandyHouTex

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Posted 10 July 2017 - 03:00 PM

SCTs have been made two ways:

 

Make a whole bunch of mirrors and a whole bunch of correctors and a whole bunch of secondaries.

Put them together and see if it works.  If not, change one of the components (usually the secondary).

Meade did this with most of their SCTs.

 

or

 

Make the mirrors and corrector, put them together, figure out what changes have to be made to the optical surfaces to yield a better image,

pull the scope apart and do hand figuring on the secondary mirror and reassemble.

Celestron did this with their earlier SCTs made in the US.  Since going to China, I doubt this happens.

 

Both techniques have worked.

 

And IF:

--the baffles are exactly perpendicular to the rear cell plate (check with a sight tube)

--the primary is centered in the tube (remove from cell and check)

--the corrector is centered in its cell (easy to measure)

--the secondary cell is centered in the corrector (easy to measure)

--the secondary is centered in its cell (easy to measure)

THEN:

--the image quality will not change with rotation of the corrector, secondary cell, or secondary.

--a laser beam from the rear opening will return exactly on itself.

 

What are the odds?

I don't know, but I haven't yet seen an SCT in which the IFs are all correct.

 

Here's the thing, though.  If the scope is made like my first technique example, you can make all the corrections yourself and improve the images.

If it's made like the second example, everything may need to be exactly as is to yield the best images.

It's my understanding Celestron still assembles their OTAs in California.  At least the Edge HDs.  Their Edge HDs are double-pass collimated and tested after assembly.  It sounds in the white paper that they still do the correctors in California too.  I'm not sure about the regular SCTs they make.  It has been said that the tri-lobed backs are sourced to China.  I have a couple and they're excellent.  The white paper also shows them working on the secondaries to touch things up.



#40 TG

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Posted 12 July 2017 - 11:46 AM

Ok. My 2 cents worth.

I have a couple of spare C8s that I experiment with. When this d--- hot weather breaks, a friend and I are going to do a more careful study of these things. But, the one thing I have already done is the following.

I happened to have the corrector off one of them for cleaning one day, and I decided to try spinning it to different positions to see what, if any, changes occurred. This was done in the daytime, with a stop sign a block away to view. So this is just what I found visually.

I placed the corrector ring back on, just tight enough to let the corrector spin, by holding the front of the secondary mirror cell. So keep in mind that the secondary was turning with the corrector.

Starting in the original position, with the corrector etching at 3:00, the view was fine. Spinning it clockwise, the view worsened the closer I got to 6:00, then started back to being right with the etching at 9:00. Continuing the spin, it got worse by 12:00, and then the view again became perfect when I got back to 3:00. In other words, it is bad at 90 degrees off either way, but good at 180 degrees with the etching exactly opposite from original position. By the way, at 6:00 and 12:00, the view was obviously distorted.

Celestron claims that they made any necessary figure corrections on the secondary mirror. If this is true, and both mirrors are suppose to be figured spherical, that would explain the changes at 90 degrees....right?

When we get back to these experiments this fall, we will try spinning the corrector plate, while leaving the secondary mirror in it's correct orientation. I am betting that moving just the corrector will not visually change the view. We will also flip the corrector front to back to see what happens, and do some corrector swapping between scopes, etc.

The effect of rotating the corrector/secondary is not that obvious. For example, it took Roddier testing for my USA vintage scope to show the difference between different secondary positions.

I suspect that you did not recollimate after rotating. Because the secondary need not be perfectly centered, rotating it throws off collimation, which the results in degraded images.

Recently somebody offered what I think is the right explanation: the corrector is not cored perfectly which results it in being a little off centered. Such a corrector, when rotated will have changing effect on optical correction with one position being the best. This however is a subtle effect and in John Hayes' article, it took an interferometer to detect.

Tanveer

#41 BillHarris

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Posted 31 May 2018 - 02:27 AM

To add to the fact of the unimportance of the plate's facing direction, it can even be tilted a couple of degrees to little detriment to the image. (The secondary mirror, naturally, must not be tilted so. wink.gif )

 

 

In fact, it should be tilted a little to avoid ghosting.

 

Mladen

 

Sorry to bring up an old thread, but use an 8" Wright Newtonian.  I took a lot of care to make sure that the corrector was aligned with the optical axis and I do have ghosting.   I'll try tilting the corrector.

 

Example: the reflection is to the right of The Horsehead.

 

 

i434_.jpg



#42 MKV

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Posted 31 May 2018 - 03:24 AM

Good! That's definitely a ghost reflection.



#43 Eddgie

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Posted 31 May 2018 - 07:27 AM

The only time that it could make a difference (though one that could be a great struggle to see) is if it is an older scope with no coatings on the "Back" of the corrector.

 

This would only be very early models though.

 

I do not know if the Starbright models used the exact same coating on both sides of the corrector. It is possible that the back side was not multicoated while I believe the front side was.

 

Again, I do not know that one would easily see the difference but unless the coatings are identical on both sides, there could be s slight change in transmission but worst case it  would probably be only a few percent. 

 

Still, with the light loss in these scopes, I would want to get it right if I could because it would be free to do so.



#44 BillHarris

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Posted 31 May 2018 - 09:58 AM

My 8" Wright is an ATM project I did about 30 years ago. The corrector is not AR coated. The reflection hadn't bothered me-- I looked at it as just part of the 'scope. Rather the "signature of a Schmidt".
But I'm refurbing the 'scope for a round of CCD astrophotography and I might as well take care of loose ends. Original coatings on the primary and diagonal, so it would be a good idea to realuminize them. And as long as I'm at it, get the corrector AR coated.
I need to start checking around for coating shops. I'm open to recommendations.

#45 GlennLeDrew

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Posted 31 May 2018 - 10:04 PM

Bill,

The tilt to apply to the corrector depends on the angular field covered. You are striving to prevent a ghost from some bright image at one extreme corner of the sensor from being formed at the opposite corner. Given that the corrector is near enough to flat, and that in reflectance the angle of reflection equals the angle of incidence, the tilt needs to be a little larger in angle than the field radius under consideration.

 

If the angular coverage from field center to one corner were to be, say, 0.9 degrees, then the corrector must be tilted by just over 0.9 degrees to the optical axis. A little more tilt than this minimum gives an extra margin against reflections from the peripheral regions surrounding the sensor--if deemed a potential source. And it helps to keep out parts of unfocused reflections from such inside-of-focus elements as, e.g., filters.



#46 BillHarris

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Posted 01 June 2018 - 12:10 AM

A 35mm frame has a diagonal of 43mm, or a width of 3.12*. Which is a radial offset of some 1.6* (rounded up). Which isn't that bad. Works out to about .25" over an OD of 9.5" on the corrector cell.

Edited by BillHarris, 01 June 2018 - 12:23 AM.


#47 davidc135

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Posted 01 June 2018 - 07:00 AM

My 8" Wright is an ATM project I did about 30 years ago. The corrector is not AR coated. The reflection hadn't bothered me-- I looked at it as just part of the 'scope. Rather the "signature of a Schmidt".
But I'm refurbing the 'scope for a round of CCD astrophotography and I might as well take care of loose ends. Original coatings on the primary and diagonal, so it would be a good idea to realuminize them. And as long as I'm at it, get the corrector AR coated.
I need to start checking around for coating shops. I'm open to recommendations.

Did you make the optics for this Wright? If so, is there an account of your progress online?

 

David



#48 Starman1

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Posted 01 June 2018 - 12:40 PM

I had a circular reflection in an SCT that was traced to not the corrector plate but the very front end of the primary baffle (shiny black, and flat cut) reflecting light into the image, creating a modestly bright circle

about 1/2° in diameter.

Flocking the tube and baffles and optimizing for contrast made the circle of light even more visible because the background became a lot darker.

What finally tamed it was using some extremely dark flat black paint on the end of the baffle.

After seeing the nature of the problem, I recommended to the manufacturers (they paid no attention) that they round the end of the baffles, and leave the end surface very scuffed up so the

anodizing would be a matte finish.



#49 BillHarris

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Posted 03 June 2018 - 11:34 AM

Did you make the optics for this Wright? If so, is there an account of your progress online?

 

David

These optics were made in the early 1980's, pre-Internet, though I did keep notes in notebooks.

This timeframe was during the lead-in to Comet Halley and I didn't have time for a major optical fabrication project, in addition to everything else.  I farmed out the optics to Paul Jones, an optician in Flagstaff, AZ.

As it turned out, that was a very good move.



#50 davidc135

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Posted 03 June 2018 - 12:18 PM

These optics were made in the early 1980's, pre-Internet, though I did keep notes in notebooks.

This timeframe was during the lead-in to Comet Halley and I didn't have time for a major optical fabrication project, in addition to everything else.  I farmed out the optics to Paul Jones, an optician in Flagstaff, AZ.

As it turned out, that was a very good move.

O.K. Thanks for the info. David




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